Recently, as power supplies for driving portable electronic equipment, such as cell phones, portable personal computers, and portable music players, and further, as power supplies for hybrid electric vehicles (HEVs) and electric vehicles (EVs), nonaqueous secondary batteries represented by lithium ion secondary batteries having a high energy density and high capacity are widely used.
As for the positive electrode active material in these nonaqueous secondary batteries, one of or a mixture of a plurality of lithium transition-metal composite oxides represented by LiMO2 (where M is at least one of Co, Ni, and Mn), (namely, LiCoO2, LiNiO2, LiNiyCo1-yO2 (y=0.01 to 0.99), LiMnO2, LiMn2O4, LiCoxMnyNizO2 (x+y+z=1)), phosphoric acid compounds having an olivine structure such as LiFePO4, and the like, all of which can reversibly absorb and desorb lithium ions, is used.
Carbonaceous materials such as graphite and amorphous carbon are widely used as the negative electrode active material in nonaqueous secondary batteries. The reason is that carbonaceous materials have discharge potential equal to that of a metal lithium or a lithium alloy but do not cause dendrite growth, and thus, carbonaceous materials have superior characteristics of high safety, superior initial efficiency, good potential flatness, and high density.
As a nonaqueous solvent for a nonaqueous electrolyte, carbonates (also referred to as carbonic acid esters), lactones, ethers, esters, and the like are used alone or in mixtures of two or more. Among them, carbonates are widely used because they have an especially high dielectric constant and provide larger ion conductivity to the nonaqueous electrolyte. A gel electrolyte, a so-called polymer electrolyte, is used as the nonaqueous electrolyte, particularly in a nonaqueous secondary battery using a laminated outer body for securing safety for liquid leakage (see JP-A-2007-305453). Hereinafter, a nonaqueous secondary battery using a laminated outer body and a polymer electrolyte is referred to as a “lithium ion polymer battery”.
JP-A-2007-305453 discloses that a coupling agent such as a silane coupling agent, an aluminum coupling agent, and a titanium coupling agent is added to an active material mixture slurry in a lithium ion polymer battery. JP-A-2007-242303 discloses an example in which a positive electrode active material is treated with a silane coupling agent having a plurality of bonding groups in order to improve cycle characteristics when intermittent cycles of a nonaqueous secondary battery are repeated. JP-A-09-199112 discloses an example in which a positive electrode mixture is mixed with an aluminum coupling agent in order to improve cycle characteristics when a nonaqueous secondary battery is charged and discharged at high voltage under heavy load.
JP-A-2002-319405 discloses an example in which a silane coupling agent having an organic reactive group such as an epoxy group and amino group and a bonding group such as a methoxy group and ethoxy group is dispersed in a positive electrode mixture in order to improve wettability of a positive electrode with a nonaqueous electrolyte in a nonaqueous secondary battery at low temperature and to improve output characteristics at low temperature. Furthermore, JP-A-2007-280830 discloses an example in which a silane coupling agent is present near a broken surface of a positive electrode active material occurring when a positive electrode mixture layer is compressed in order to improve cycle characteristics of a nonaqueous secondary battery.
In the invention disclosed in JP-A-2007-305453, adhesiveness between an electrode and a gel electrolyte is improved and thus a lithium ion polymer battery in which an increase in battery resistance and an increase in thickness are suppressed can be obtained. However, JP-A-2007-305453 does not describe improvement of nail penetration characteristics of a lithium ion polymer battery. The nail penetration characteristics show the degree of smoking or burning when a nail penetrates a battery. In the nail penetration characteristics test, a forced internal short circuit is caused inside a battery, and thus, the temperature becomes abnormally high locally inside the battery. Therefore, nail penetration characteristics are one indicator that shows the degree of battery thermal runaway.
The inventions disclosed in JP-A-2007-242303, JP-A-09-199112, JP-A-2002-319405, and JP-A-2007-280830 show that mixing a silane or aluminum coupling agent in a positive electrode mixture can possibly lead to an improvement in cycle characteristics and output characteristics in a low temperature environment to some extent. However, JP-A-2007-242303, JP-A-09-199112, JP-A-2002-319405, and JP-A-2007-280830 provide no description on nail penetration characteristics of nonaqueous secondary batteries in which a silane or aluminum coupling agent is mixed in a positive electrode mixture.
The inventors of the invention have carried out various studies in order to ensure cycle characteristics and nail penetration characteristics of a lithium ion polymer battery using such a gel electrolyte. As a result, the inventors have found that the problems mentioned above can be solved when a positive electrode mixture contains a predetermined amount of an aluminum coupling agent, the average particle diameter and the specific surface area of the positive electrode active material including a lithium composite oxide are maintained in a predetermined range, and a gel nonaqueous electrolyte formed from a nonaqueous electrolyte containing a monomer having a (meth)acrylic end group is used, whereby the invention has been achieved.